The FANC authorises restart of Doel 3 and Tihange 2 reactors

PRESS RELEASE
Brussels, 17th November 2015
The FANC authorises restart
of Doel 3 and Tihange 2 reactors
On 17 November 2015, the Federal Agency for Nuclear Control (FANC) authorised
the licensee Electrabel to restart the Doel 3 and Tihange 2 reactor units. Both
reactors have been shut down for some time now because of concerns about
their safety.
The uncertainty about the safety of these reactors was due to the discovery of thousands of
flaw indications in the steel walls of their reactor pressure vessels, during their scheduled
outage in the summer of 2012. Further investigation revealed that these flaw indications had
been caused by hydrogen bubbles. These bubbles had become trapped in the material when
the steel rings for the pressure vessel were manufactured and then rolled out into hydrogen
flakes with an average length of 12 to 16 millimetres and the thickness of a cigarette paper.
These cracks are known as hydrogen flakes. The hydrogen flakes are laminar, which means
they run parallel with the wall of the pressure vessel and so do not actually ‘pierce’ it.
After these cracks were discovered, the FANC decided to prohibit Electrabel from restarting
the Doel 3 and Tihange 2 reactors until it could prove conclusively that the safety of the
reactors was not at risk due to the presence of the hydrogen flakes. The evidence for this
had to be provided in the form of a Safety Case for each reactor. These Safety Cases would
be assessed by the FANC and various teams of national and international experts.
In December 2012, Electrabel submitted the first Safety Case reports, which were later
supplemented with addenda. In May 2013, on the basis of the evidence provided in these
documents and the recommendations of independent experts, the FANC authorised
Electrabel to restart both reactors. A condition was that the licensee was required to
complete a series of studies and measures before the end of the first reactor cycle, i.e.
within approx. 1 year of the restart.
In March 2014, Electrabel reported that one of these compulsory studies, the fracture
toughness test, had produced unexpected results. As a precautionary measure, Doel 3 and
Tihange 2 were subsequently shut down again. The FANC required the licensee to submit a
new Safety Case for both reactors to justify a restart.
These Safety Case reports were once again submitted to a large number of national and
international experts, including the US Oak Ridge National Laboratory. The FANC collected
together and analysed the reports and recommendations of the various teams of experts in
order to come to its own conclusions.
The FANC concludes that Electrabel has demonstrated that the hydrogen flakes in the walls
of the reactor vessels do not pose an unacceptable safety risk for the reactors. Electrabel is
therefore authorised to restart and operate the Doel 3 and Tihange 2 reactors until they are
FANC – Ravensteinstreet 36, 1000 Brussels – www.fanc.fgov.be
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permanently decommissioned. By law this is planned on 1 October 2022 for Doel 3 and on 1
February 2023 for Tihange 2.
The FANC wishes to emphasise that both the Safety Case reports submitted by Electrabel
and the final decision to authorise the restart are based on extensive scientific studies.
These studies have provided us with certainty that the hydrogen flakes in the reactor vessels
of Doel 3 and Tihange 2 have been correctly detected and sized, and that the presence of
hydrogen flakes does not jeopardise the safety of the nuclear plants.
The file on Doel 3 and Tihange 2 is highly complex and difficult to explain in a standard brief press
release. For this reason, we have provided a more detailed chronological overview and scientific
background to the case below. Although this is by no means a comprehensive overview, it may help
you to understand this case better. Should you have any further questions after reading this
overview, please contact the FANC spokespersons:
Nele Scheerlinck (NL, ENG): +32 (0)2 289 20 17
Sébastien Berg (FR, NL): +32 (0)2 289 20 30
Dirk Vandriessche (ENG, DU): +32 (0)2 289 20 34
You can also find more information on our website: www.fanc.fgov.be. All news and press releases
that the FANC has published on this case can be found on the overview page. There is also an
extensive FAQ and you can find all other documents that the FANC has published related to this case:
Electrabel’s Safety Cases, FANC’s evaluation reports and the reports and recommendations of the
experts we consulted.
FANC – Ravensteinstreet 36, 1000 Brussels – www.fanc.fgov.be
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Case chronology and scientific background
Summer 2012: discovery of flaw indications
In the summer of 2012, the reactor pressure vessel of Doel 3 was inspected using ultrasonic
probes as part of a scheduled safety review. The aim of such an ultrasonic inspection is to
test the reactor pressure vessel for the presence of ‘under-clad defects’ (gaps between the
steel and inox inner surface of the reactor pressure vessel). No under-clad defects were
found, but the ultrasonic test did reveal other unexplained flaws inside the steel wall of the
reactor pressure vessel. It was subsequently decided to prohibit Doel 3 from restarting until
the nature and source of the flaw indications was identified.
In September 2012, the reactor pressure vessel of Tihange 2, that was manufactured by the
same company (Rotterdamsche Droogdokmaatschappij), was subjected to the same
ultrasonic testing. Tihange 2 proved to have similar flaw indications to Doel 3.
The flaws were soon revealed to be hydrogen flakes. When steel components are
manufactured, there is a risk of excess hydrogen building up in the cast steel during the
process of cooling and curing. When the hydrogen evaporates it leaves behind tiny bubbles
in the steel. In this case, the bubbles were rolled flat into “hydrogen flakes” during the
forging process, with an average length of 12 to 16 mm and the thickness of a cigarette
paper. The hydrogen flakes are laminar, which means they run parallel to the curve of the
wall of the reactor pressure vessel.
The occurrence of hydrogen flakes in steel structures is a well-known and widely studied
phenomenon in metallurgy. However, this was the first time the phenomenon had been
observed in the walls of the reactor pressure vessel of a nuclear power plant in operation.
Following this discovery, the FANC ordered that the Doel 3 and Tihange 2 nuclear reactors
were not to be restarted until Electrabel had conclusively proved that the presence of the
hydrogen flakes did not form a risk for the integrity and hence the safety of the reactor
pressure vessels.
May 2013: The FANC authorised restart
Following a series of extensive studies, Electrabel submitted two Safety Case reports in
December 2012, in which it argued that the safety of Doel 3 and Tihange 2 was not at risk
due to the presence of hydrogen flakes in the walls of the pressure vessels. In January
2013, the FANC concluded that there was no cause for permanent shutdown of these plants,
however they did require that Electrabel provide them with additional information. The
licensee subsequently submitted two addenda to the Safety Case reports.
In May 2013, after carefully analysing the case documents and consulting with various
national and international experts, the FANC concluded that the evidence provided by
Electrabel was sufficient to guarantee the safe operation of the Doel 3 and Tihange 2
reactors. On 17 May 2013, the FANC authorised the restart of the reactors.
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T : +32 (0)2 289 21 11 – F : +32 (0)2 289 21 12 – E : [email protected]
However, they did set a number of additional conditions on this authorisation. Among
others, the licensee was required to complete a series of actions before the end of the first
reactor cycle (approx. 1 year after the restart).
March 2014: fracture toughness test produced unexpected results
One of the measures that Electrabel had to complete before the end of the first reactor cycle
involved mechanical testing of irradiated steel components affected by hydrogen flakes. The
aim of these tests was to find out to what extent exposure to intensive radiation influences
the mechanical properties of steel that contains hydrogen flakes. Electrabel was required to
conduct these tests in order to empirically confirm some hypotheses and calculations in its
Safety Case reports.
The tests used samples provided by a French steam generator (“VB-395”) that was
constructed for use in a nuclear power plant but was rejected because the steel contained
hydrogen flakes. The samples were first placed in the research reactor of the Belgian
Nuclear Research Centre (SCK•CEN) in Mol, where they were intensively irradiated for four
weeks in order to simulate the cumulative radiation exposure of 40 years of operation. The
samples were then subjected to a series of tests to evaluate their mechanical properties. All
the tests produced the expected results, with the exception of one test: the fracture
toughness test. The results of this test suggested that the steel of the VB-395 specimen
would become brittle much faster than expected based on theory.
Because the researchers did not have an immediate explanation for these unexpected test
results, it was decided on 25 March 2014 to shut down the Doel 3 and Tihange 2 reactors
again as a precautionary measure. The FANC ordered that the reactors were not to be
restarted until Electrabel had conclusively proved that the conclusion of 2013 remains valid
(the presence of hydrogen flakes has no negative impact on the safety of the installations)
despite the unexpected test results. During the following months, Electrabel commissioned
SCK•CEN to conduct a number of new tests.
Because the study of the mechanical properties of irradiated materials is a highly specialised
research domain, the FANC called in the help of a number of leading international experts in
this area to review this particular aspect of the Safety Case. This team of experts (the
International Review Board) was asked to recommend an appropriate methodology to
Electrabel for those parts of the Safety Cases that were related to the mechanical properties
of steel under the influence of radiation.
February 2015: number of detected hydrogen flakes re-evaluated
Alongside the mechanical tests, Electrabel was also required to complete a number of other
actions. A number of these actions were related to the qualification of the ultrasonic testing
method with which the hydrogen flakes were discovered in the summer of 2012. Electrabel
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was required to provide more evidence that the testing method used was capable of
detecting and accurately sizing all the hydrogen flakes.
The new tests revealed that the ultrasonic inspection method had indeed identified most of
the flaw indications, but that the detection thresholds needed to be adjusted in order to be
able to guarantee that literally every hydrogen flake was detected. It was also concluded
that the method used to interpret the test results left room for improvement.
In 2014, new ultrasonic tests were conducted on the reactor pressure vessels and the
results were interpreted using an alternative method. This led to the detection of some 60%
more flaw indications than during the previous inspection, especially thanks to lowering the
detection threshold. With thus new inspection method, the researchers also noted an
increase in the average and maximum length of the falkes. This was because the new
interpretation method prescribed that hydrogen flakes that were close together were to be
treated as single elongated flake.
The FANC called in the expertise of the certified inspection company AIB-Vinçotte to
evaluate the results of the qualification of the ultrasonic testing method.
July 2015: Electrabel submitted new Safety Case reports to the FANC
On 17 July 2015, Electrabel submitted the new Safety Case reports for Doel 3 and Tihange 2
to the FANC.
As required by the FANC, these Safety Case reports focussed on three major themes,
whereby the research results of themes 1 and 2 were to support the evidence for theme 3:
1. Detecting, measuring and locating the flaw indications by means of ultrasonic tests;
2. The mechanical properties of the material containing hydrogen flakes and the
evolution of these mechanical properties under irradiation;
3. The structural integrity of a reactor pressure vessel that contains hydrogen flakes.
The FANC forwarded copies of the Safety Case reports to the external experts involved in
the definitive safety review. The certified inspection company AIB-Vinçotte wrote an
evaluation report on the first theme and the Oak Ridge National Laboratory (USA) was
commissioned to conduct a thorough and independent evaluation of all hypotheses, the
methodology, the calculations and the interpretation of the results and to re-do all
calculations, based on its own computer codes, hypotheses and methodology. Bel V, the
FANC's technical branch, conducted a separate evaluation of the entire Safety Case. Finally,
a working group of four Belgian professors considered the question of whether the hydrogen
flakes might grow due to the migration of hydrogen through the walls of the reactor
pressure vessel.
The reports of the various external experts were sent to the FANC over the following
months and compared with the conclusions of the FANC’s own experts.
FANC – Ravensteinstreet 36, 1000 Brussels – www.fanc.fgov.be
T : +32 (0)2 289 21 11 – F : +32 (0)2 289 21 12 – E : [email protected]
November 2015: the FANC authorises the restart of Doel 3 and Tihange 2
Based on its own review and the reports of the various external experts, the FANC
concluded that there was no cause anymore to prevent the Doel 3 and Tihange 2 reactors
from restarting.
Electrabel was able to demonstrate that the unexpected test results of March 2014 were
probably due to the specific material properties of the specimen that was used. Tests on
another material with hydrogen flakes and on the material of the reactor pressure vessels
themselves demonstrated that long-lasting irradiation will not have an abnormal impact on
the mechanical properties of the reactor pressure vessels of Doel 3 and Tihange 2.
The structural integrity of the reactor pressure vessels of Doel 3 and Tihange 2 complies
with the safety requirements and the presence of hydrogen flakes does not pose a risk to
the safety of the nuclear power plants.
FANC – Ravensteinstreet 36, 1000 Brussels – www.fanc.fgov.be
T : +32 (0)2 289 21 11 – F : +32 (0)2 289 21 12 – E : [email protected]